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<p class="level0"><a name="NAME"></a><h2 class="nroffsh">NAME</h2>
<p class="level0">pcap_compile - compile a filter expression <a name="SYNOPSIS"></a><h2 class="nroffsh">SYNOPSIS</h2>
<p class="level0"><pre class="level0">
&#35;include &lt;pcap/pcap.h&gt;
int pcap_compile(pcap_t *p, struct bpf_program *fp,
&nbsp;&nbsp;&nbsp;&nbsp;const char *str, int optimize, bpf_u_int32 netmask);
</pre>

<p class="level0"><a name="DESCRIPTION"></a><h2 class="nroffsh">DESCRIPTION</h2>
<p class="level0"><span Class="bold">pcap_compile()</span> is used to compile the string <span Class="emphasis">str</span> into a filter program.  See <a Class="bold" href="./pcap-filter.html">pcap-filter</a>(7) for the syntax of that string. <span Class="emphasis">program</span> is a pointer to a <span Class="emphasis">bpf_program</span> struct and is filled in by <span Class="bold">pcap_compile()</span>. <span Class="emphasis">optimize</span> controls whether optimization on the resulting code is performed. <span Class="emphasis">netmask</span> specifies the IPv4 netmask of the network on which packets are being captured; it is used only when checking for IPv4 broadcast addresses in the filter program.  If the netmask of the network on which packets are being captured isn&#39;t known to the program, or if packets are being captured on the Linux &quot;any&quot; pseudo-interface that can capture on more than one network, a value of <span Class="bold">PCAP_NETMASK_UNKNOWN</span> can be supplied; tests for IPv4 broadcast addresses will fail to compile, but all other tests in the filter program will be OK. 
<p class="level0">NOTE: in libpcap 1.8.0 and later, <span Class="bold">pcap_compile()</span> can be used in multiple threads within a single process.  However, in earlier versions of libpcap, it is <span Class="emphasis">not</span> safe to use <span Class="bold">pcap_compile()</span> in multiple threads in a single process without some form of mutual exclusion allowing only one thread to call it at any given time. <a name="RETURN"></a><h2 class="nroffsh">RETURN VALUE</h2>
<p class="level0"><span Class="bold">pcap_compile()</span> returns 0 on success and <span Class="bold">PCAP_ERROR</span> on failure. If <span Class="bold">PCAP_ERROR</span> is returned, <a Class="bold" href="./pcap_geterr.html">pcap_geterr</a> or <span Class="bold">pcap_perror(3PCAP)</span> may be called with <span Class="emphasis">p</span> as an argument to fetch or display the error text. <a name="BACKWARD"></a><h2 class="nroffsh">BACKWARD COMPATIBILITY</h2>
<p class="level0">
<p class="level0">The <span Class="bold">PCAP_NETMASK_UNKNOWN</span> constant became available in libpcap release 1.1.0. <a name="SEE"></a><h2 class="nroffsh">SEE ALSO</h2>
<p class="level0"><a Class="bold" href="./pcap.html">pcap</a>(3PCAP), <a Class="bold" href="./pcap_setfilter.html">pcap_setfilter</a>(3PCAP), <a Class="bold" href="./pcap_freecode.html">pcap_freecode</a>(3PCAP) <p class="roffit">
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